Fluid extraction

ABSTRACT

A method of extracting metalloid and metal species from a solid or liquid material by exposing the material to a supercritical fluid solvent containing a chelating agent is described. The chelating agent forms chelates that are soluble in the supercritical fluid to allow removal of the species from the material. In preferred embodiments, the extraction solvent is supercritical carbon dioxide and the chelating agent is a fluorinated β-diketone. In especially preferred embodiments the extraction solvent is supercritical carbon dioxide, and the chelating agent comprises a fluorinated β-diketone and a trialkyl phosphate, or a fluorinated β-diketone and a trialkylphosphine oxide. Although a trialkyl phosphate can extract lanthanides and actinides from acidic solutions, a binary mixture comprising a fluorinated β-diketone and a trialkyl phosphate or a trialkylphosphine oxide tends to enhance the extraction efficiencies for actinides and lanthanides. The method provides an environmentally benign process for removing contaminants from industrial waste without using acids or biologically harmful solvents. The method is particularly useful for extracting actinides and lanthanides from acidic solutions. The chelate and supercritical fluid can be regenerated, and the contaminant species recovered, to provide an economic, efficient process.

ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT

This invention was funded in part by (1) the United States Department ofEnergy, under Special Master Task research, Subcontract No. C85-110554,(2) a grant from DOE Idaho Field Office under the Office of TechnologyDevelopment's Innovative Technology Department Program and by NSF-IdahoEPSCOR Program under NSF Cooperative Agreement OSR-9350539, and (3) theNational Science Foundation, under Grant RII-8902065. The United StatesGovernment may have certain rights in this invention as a result ofthese grants.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of United States Patentapplication, Ser. No. 07/781,086, filed Oct. 21, 1991, now U.S. Pat. No.5,356,538, which is a continuation-in-part of United States Patentapplication, Ser. No. 07/714,265, now U.S. Pat. No. 5,274,129, filedJune 12, 1991.

FIELD OF THE INVENTION

This invention concerns extraction of metalloids and metals from solidsand liquids, and is more particularly directed to a treatment process inwhich metals are efficiently extracted from waste material.

BACKGROUND OF THE INVENTION

Waste treatment and disposal is an important social and economic issue.Industries throughout the world spend large sums of money to reduce thebiological hazards of environmental exposure to toxic substances. Oneparticular environmental problem is the removal of toxic metals andradioisotopes from solid or liquid industrial wastes. Such contaminantscan be removed from soils, for example, by treating the soil with anacid that dissolves the metals. Acid dissolution is followed byselective precipitation, electrowinning, or solvent extraction. Aciddissolution is unfortunately very nonspecific, and often produces manyby-products that can create serious environmental problems in their ownright.

An alternative detoxification process is to encapsulate contaminants ina container or insoluble matrix that prevents their entry into theenvironment. This approach still requires storage of the bulky matrix,and does not allow regeneration or reuse of the contaminants. Hencethere is a need for a biologically compatible waste treatment processthat efficiently and effectively separates metals from contaminatedmaterials. There is also a need for such a process that is biologicallycompatible and permits selective regeneration and reuse of thecontaminants.

One of the present inventors previously has disclosed that enhancedextraction of metals can be achieved with ionizable crown ethers, suchas crown ether carboxylic acids. The inventors have found that thesemacrocyclic ethers have cavities that can selectively extractlanthanides and actinides by attracting these species with an ionizedside chain.

The metal ion is then inserted into the cavity of the macrocycle to forma chelate. Analyst, 114:451-453 (1989) and Anal. Chem 58:3233-3235(1986). This mechanism of attracting the ion and inserting it in thering has earned these compounds the name of "lariat crown ethers." Inthese reports, a crown ether carboxylic acid(sym-dibenzo-16-crown-5-oxyacetic acid) was used to extract lanthanidesfrom aqueous solutions into an organic phase with high efficiency andselectivity. U.S. Pat. No. 4,908,135 similarly discloses separation ofsecondary and tertiary amines using a different crown ether, while U.S.Pat. No. 4,942,149 shows separation of racemic compounds with yet othercrown ethers.

An unrelated solvent extraction method is fluid extraction, such assupercritical fluid extraction. A supercritical fluid is typically onethat is gaseous at ambient conditions, but which is maintained at atemperature and pressure above its critical temperature and pres sure.Although materials may p erform as solvents at sub-critical temperaturesand pressures, fluids often perform better as solvents at supercriticalconditions. Supercritical solvents can be used to ex tract organicmaterials such as caffeine from coffee beans. U.S. Pat. No. 4,911,941provides an example of supercritical carbon-dioxide extraction ofcaffeine in which green coffee beans are moved periodically through anextraction vessel and contacted with continuously flowing supercriticalcarbon dioxide. U.S. Pat. No. 4,898,673 shows a similar system in whichsoluble materials are co ntinuously extracted from solids usingsupercritical carbon dioxide. The soluble solids are circulated in aclosed-loop pipeline with the supercritical fluid.

Supercritical extraction of environmental wastes has not previously beensuggested. This may be due to the relatively low solubility of metalsand other non-organic materials in supercritical fluids. Directextraction of metal ions by supercritical carbon dioxide, for example,is inefficient because of the weak van der Waals interaction betweenmetal ions and carbon dioxide. This weak interaction has apparentlydiscouraged efforts to perform supercritical fluid extraction of metalsfrom environmental wastes.

SUMMARY OF THE INVENTION

The present invention provides a method of extracting a metal species(including lanthanides and actinides) from a solid or liquid by exposingthe solid or liquid to a fluid solvent, particularly a supercriticalfluid solvent, that contains a chelating agent. The fluid orsupercritical fluid solvent and chelating agent are exposed to the solidor liquid for a sufficient period of time to form a chelate between themetal and chelating agent that is soluble in the fluid. The fluid orsupercritical fluid then is removed from the solid or liquid with thesolubilized metal chelate dissolved in the fluid. The metal chelatessubsequently can be precipitated from the fluid. For example, if thefluid is supercritical, then the metal chelates can be precipitated byreducing the pressure of the supercritical fluid. The chelating agentcan also be regenerated for reuse. The resulting process is anefficient, cost-effective method for removing metals from theenvironment without using environmentally harmful extraction solvents.

The chelating agents can be any agent that forms a chelate with themetal being extracted, wherein the chelate is soluble in the fluid orsupercritical fluid solvent. Examples of suitable chelating agentsinclude dithiocarbamates, ionizable crown ethers, β-diketones andtrialkyl phosphates, as shown below. ##STR1##

In especially preferred embodiments, the chelating agent is fluorinatedto enhance the solubility of the metal chelate in supercritical carbondioxide. Examples of fluorinated chelating agents are: ##STR2## Thesolubilities of some halogenated metal chelates in supercritical carbondioxide, and in particular the fluorinated metal chelates, are enhancedby two to three orders of magnitude relative to the correspondingnon-fluorinated metal chelates. For instance, the solubility ofCu(FDDC)₂ in supercritical carbon dioxide is about 1×10⁻³ moles perliter at 50° C. and 100 atmospheres, whereas the solubility of thenon-fluorinated compound, Cu(DDC)₂, is less than 1×10⁻⁶ moles per literunder the same conditions. Fluorinated chelating agents have been foundto greatly enhance the efficiency of metal extraction in supercriticalcarbon dioxide. As a result, fluorinated chelating agents currently arepreferred chelating agents useful for practicing the present invention.

In yet other embodiments of the invention, a modifier is added to thesupercritical fluid to further enhance the efficiency of the extractionmethod by increasing the solubility of the metal chelate in thesupercritical fluid. Carbon dioxide, for example, is a relativelynon-polar solvent. Its polarity can be increased by adding a more polarsolvent to the supercritical carbon dioxide. Disclosed examples of morepolar solvents are low to medium boiling point alcohols or esters, suchas methanol. The alcohol or ester increases the polarity of thesupercritical fluid, enhances the solubility of the metal chelate in thefluid, and further improves the extraction efficiency of the method.

The present method also can be used to selectively remove particularcontaminants from liquid or solid waste. Ionizable crown ethers of agiven ionic diameter can, for example, selectively remove lanthanidesand actinides from the waste material. Suitable crown ethers includedibenzo crown ether derivatives of a hydroxamic acid represented by theformula: ##STR3## wherein X is a dibenzo crown ether of the formuladibenzo [13+3m]-crown-[4+m] ether, and m is an integer of from 0 toabout 5; n is an integer from 0 to 6; and R₁ is H or a lipophilichydrocarbyl group having from 1 to about 18 carbon atoms that isselected from the group consisting of alkyl, cycloalkyl, alkenyl andaryl groups. In more preferred embodiments, the ionizable crown etherhas the chemical formula ##STR4## wherein X is OH or NHOH; R₂ is alkyl,fluorinated alkyl, phenyl or fluorinated phenyl; R₃ is alkyl,fluorinated alkyl, phenyl or fluorinated phenyl; R₄ is H or F; R₅ is Hor F; and n is 1 to 3. When n is 1, the chelating agent is ##STR5##

In yet other embodiments, the chelating agent is a dithiocarbamatehaving the general formula ##STR6## wherein R₆ and R₇ are independentlyalkyl or aromatic groups that may contain one or more fluorine atoms.When R₆ =R₇ =CH₃ CH₂, the ligand is called diethyldithiocarbamate (DDC),and it forms a metal chelate such as ##STR7## where M is a metal.

In particularly preferred embodiments, R₆ and R₇ are both CF₃ CH₂, theligand is called bis(trifluoroethyl)dithiocarbamate, and the resultingmetal chelate has a structure such as ##STR8## where M is a metal.

In more particular embodiments of the invention, a system is providedfor treating waste material containing metal species. The material isplaced in a container through which the fluid or supercritical fluid ispassed to solubilize the metal species. The fluid or supercritical fluidand solubilized metal species are removed from the container to separatethe metal species from the waste material. In preferred embodiments, achelating agent is dissolved in the fluid to form chelates with themetal species that are soluble in the fluid. In especially preferredembodiments, the chelates are fluorinated to further increase theirsolubility and enhance the extraction efficiency of the separationmethod. Polar solvents such as alcohols or esters can be added to thesupercritical fluid to also enhance solubility of the metal chelate inthe supercritical fluid.

Fluid or supercritical fluid can be flowed continuously through thewaste material, or exposed to the material in a discontinuous batchprocess. In one embodiment, a supercritical fluid is flowed through achelating agent before the waste material is exposed to the fluid todissolve the chelating agent in the fluid. After the metal chelates haveformed and dissolved in the supercritical fluid, the pressure on thesupercritical fluid can be reduced to below supercritical levels suchthat the fluid becomes a gas and the metal chelates are precipitatedfrom the system. The pure metal can then be collected, and the chelatingagent recycled to further extract the waste material. The chelatingagent can be separated from the metal, for example, by 0.1 M or moreconcentrated nitric acid with a pH less than or equal to 1.

The present invention also provides a method for extracting a metalloidor metal species from a solid or liquid comprising exposing the solid orliquid to a fluid solvent, particularly a supercritical fluid,containing a β-diketone chelating agent for a sufficient period of timeto form chelates between the agent and species that are solubilized inthe fluid solvent. A preferred supercritical fluid is supercriticalcarbon dioxide. The fluid typically is separated from the solid orliquid after the chelate is solubilized in the fluid. Preferredβ-diketones include halogenated β-diketones, particularly fluorinatedβ-diketones. As with the embodiments described above, the fluid solventmay further comprise a secondary modifying solvent, such as mediumboiling point alcohols and esters, particularly methanol.

The β-diketone may be represented by the formula ##STR9## wherein R₁ andR₂ are independently selected from the group consisting of lower alkyl,fluorinated lower alkyl and thenoyl groups. As used herein, the term"lower alkyl" refers to compounds having ten or fewer carbon atoms, andincludes both straight chain and branched chain compounds. Morespecifically, R₁ may be selected from the group consisting of methyl,trifluoromethyl, ethyl, fluorinated ethyl, propyl, fluorinated propyl,butyl and fluorinated butyl, and R₂ may be selected from the groupconsisting of methyl, trifluoromethyl, ethyl, fluorinated ethyl, propyl,fluorinated propyl, butyl, and fluorinated butyl. As used herein, a"halogenated lower alkyl group", such as a fluorinated ethyl group meansthat at least one of the hydrogen atoms present on the alkyl group isreplaced with a halogen atom, preferably a fluorine atom. A "halogenatedlower alkyl group" also can refer to compounds wherein all of thehydrogen atoms have been replaced with halogens, preferably fluorineatoms. An example of such a halogenated lower alkyl group would be atrifluoromethyl group. Specific examples of suitable β-diketones includeacetylacetone, trifluoroacetylacetone, hexa-fluoroacetylacetone,thenoyltrifluoroacetylacetone and heptafluorobutanoylpivaroylmethane.Especially preferred β-diketones include trifluoroacetylacetone,hexa-fluoroacetylacetone, thenoyltrifluoroacetylacetone andheptafluoro-butanoylpivaroylmethane.

A preferred method according to the present invention comprises exposinga solid or liquid containing metal or metalloid ions to carbon dioxide,particularly supercritical carbon dioxide, containing a ligand selectedfrom the group consisting of fluorinated β-diketones, trialkylphosphates, trialkylphosphine oxides, and mixtures thereof. At least ofthe fluorinated β-diketone, trialkyl phosphate and trialkylphosphineoxides forms chelates with the metal or metalloid species. The chelatesare soluble in the supercritical carbon dioxide. The fluorinatedβdiketone may be represented by the formula ##STR10## wherein R₁ and R₂are independently selected from the group consisting of fluorinatedlower alkyl and fluorinated thenoyl groups. The trialkyl phosphate maybe represented by the formula ##STR11## wherein R₃, R₄ and R₅ areindependently selected from the group consisting of lower alkyl groups,and the trialkylphosphine oxide may be represented by the formula##STR12## wherein R₆ -R₈ are independently selected from the groupconsisting of lower alkyl groups.

More specifically, the β-diketone may be selected from the groupconsisting of trifluoroacetylacetone, hexafluoroacetylacetone,thenoyltrifluoroacetylacetone and heptafluoro-butanoylpivaroylmethane,and R₃, R₄ and R₅ may be selected from the group consisting of n-butyland n-octyl. A modifying solvent also can be used with this particularembodiment of the present invention. The modifying solvent may beselected from the group consisting of lower alkyl alcohols, with aparticular embodiment of the modifying solvent being methanol.

The present invention also provides an extraction solvent, comprising asupercritical fluid and a β-diketone chelating agent. The solventpreferably is supercritical carbon dioxide, the β-diketone preferably isa fluorinated β-diketone, and the solvent preferably further comprises atrialkyl phosphate or a trialkylphosphine oxide chelating agent.

Accordingly, it is an object of this invention to provide an improvedmethod for extracting metals from liquids or solids, including complexmatrices.

Another object of the invention to provide such an improved method thatallows efficient and biologically compatible extraction of metals fromthe environment.

Another object is to provide such an improved method that allowsselectivity as to the type of metal extracted by the system.

Another object is to provide such an improved method that canselectively extract lanthanides and actinides.

Another object of this invention is to provide such an improved methodthat is efficient and economical compared to many other extractionprocesses.

Another object of this invention is to provide a process for theselective removal of ions from acidic waste systems.

Another object of this invention to provide a process for the efficientextraction of metals and metalloids from solid and liquid materialsusing a β-diketone, particularly a fluorinated β-diketone.

Finally, it is an object of this invention to provide a process for thefluid extraction of metal and metalloids from solid and liquid materialsusing a mixed ligand composition comprising a β-diketone, particularly afluorinated β-diketone, and a trialkyl phosphate.

These and other objects of the invention will be understood more clearlyby reference to the following drawings and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a system for using supercritical fluid toextract Cu²⁺ from an aqueous solution.

FIG. 2 is a graph illustrating the rate of extraction of Cu²⁺ from waterusing supercritical CO₂ with varying densities at 35° C. saturated withbis(trifluoroethyl)dithiocarbamate (FDDC).

FIG. 3 is a chromatogram produced using a superbond capillary columninjected at 100° C. oven temperature with a hold time of 6.5 minutes at100 atm followed by a 4.0 atm/minute ramp.

FIG. 4 is a chromatogram as in FIG. 3 showing peaks for (a) NaFDDC, (b)Zn(FDDC)₂, (c) Ni(FDDC)₂, (d) Co(FDDC)₃, (e) Fe(FDDC)₃, (f) Hg(FDDC)₂,(g) As(FDDC)₃, (h) Sb(FDDC)₃, (i) Bi(FDDC)₃.

FIG. 5 is a graph comparing solubilities of an unfluorinated lariatcrown ether with fluorinated lariat crown ethers.

FIG. 6 is a schematic drawing of a waste treatment system in accordancewith the present invention.

FIG. 7 is a phase diagram for carbon dioxide.

DETAILED DESCRIPTION OF SEVERAL PREFERRED EMBODIMENTS I. GeneralDiscussion of the Invention

The present invention concerns a method for extracting metalloids ormetals from solid or liquid materials by exposing the material to afluid solvent or a supercritical fluid solvent. The fluid orsupercritical fluid preferably contains either a chelating agent thatforms a metal chelate with the extracted metal, or the fluid may includea ligand and a solubility-enhancing agent. The chelate is soluble in thefluid, particularly supercritical fluids, which allows efficientextraction of the chelate from the material.

The present invention is suitable for removing many different types ofmetalloids or metals from liquids or solids. Metalloids are elementswith both metallic and non-metallic properties, and include arsenic,selenium and tellurium. A metal is an element that forms positive ionsin solutions, and produces oxides that form hydroxides rather than acidswith water. Metals include alkali metals, alkali-earth metals,transition metals, noble metals (including the precious metals gold,platinum and silver), rare metals, rare-earth metals (lanthanides),actinides (including the transuranic metals), light metals, heavymetals, synthetic metals and radioactive metals. Specific examples aregiven herein of extraction methods for removing lanthanides andactinides (collectively referred to as the β-group elements from thefilling of their 4f and 5f orbitals) as well as transition metals suchas copper. The f group elements are commonly produced by nuclear fissionreactions, and the actinides are radioactive. Transition metals arecommonly used or produced in many industrial processes and products,such as mineral production or fly ash. The present invention alsoprovides specific examples of extraction methods for radioactive metals,such as uranium, particularly the extraction of such metals from acidicsolutions. This provides an attractive alternative to the PUREX processfor recovering uranyl ions from acidic solutions.

Suitable fluids and/or supercritical fluids for use in the presentinvention include carbon dioxide, nitrogen, nitrous oxide, methane,ethylene, propane and propylene. Carbon dioxide is a particularlypreferred fluid for both subcritical and supercritical fluid extractionsbecause of its moderate chemical constants (T_(c) =31° C., P_(c) 73 atm)and its inertness (i.e. it is non-explosive and thoroughly safe forextractions, even extractions performed at supercritical conditions).Carbon dioxide also is a preferred solvent because it is abundantlyavailable and relatively inexpensive.

FIG. 7 is a phase diagram for carbon dioxide which shows the conditionsnecessary to produce either subcritical liquid carbon dioxide orsupercritical carbon dioxide. Although all conditions above the triplepoint (T_(P)) produce a carbon dioxide fluid solvent effective forpracticing the present invention, the preferred carbon dioxide solventis supercritical. Therefore the conditions typically must be above thecritical temperature and pressure for carbon dioxide. However, virtuallyany conditions that are above the critical point are acceptable forproducing a supercritical carbon dioxide fluid solvent useful forpracticing the extraction process of the present invention.

The fluids may be used either individually or in combinations, as mixedfluids or supercritical fluid solvents. Examples of other fluids, andtheir critical temperature and pressure, are shown in the followingTable I:

                  TABLE I                                                         ______________________________________                                        PHYSICAL PARAMETERS                                                           OF SELECTED SUPERCRITICAL FLUIDS*                                                       Molecular                                                           Fluid     Formula  T.sub.c (° C.)                                                                 p.sub.c (atm)                                                                       ρ.sub.c (g/mL)                                                                   ρ400.sub.atm **                   ______________________________________                                        Carbon dioxide                                                                          CO.sub.2 31.1    72.9  0.47   0.96                                  Nitrous oxide                                                                           N.sub.2 O                                                                              36.5    71.7  0.45   0.94                                  Ammonia   NH.sub.3 132.5   112.5 0.24   0.40                                  η-Pentane                                                                           C.sub.5 H.sub.12                                                                       196.6   33.3  0.23   0.51                                  η-Butane                                                                            C.sub.4 H.sub.10                                                                       152.0   37.5  0.23   0.50                                  η-Propane                                                                           C.sub.3 H.sub.6                                                                        96.8    42.0  0.22   --                                    Sulfur    SF.sub.6 45.5    37.1  0.74   1.61                                  hexafluoride                                                                  Xenon     Xe       16.6    58.4  1.10   2.30                                  Dichloro- CCl.sub.2 F.sub.2                                                                      111.8   40.7  0.56   1.12                                  difluoromethane                                                               Trifluoromethane                                                                        CHF.sub.3                                                                              25.9    46.9  0.52   --                                    Methanol  CH.sub.3 OH                                                                            240.5   78.9  0.27   --                                    Ethanol   C.sub.2 H.sub.5 OH                                                                     243.4   63.0  0.28   --                                    Isopropanol                                                                             C.sub.3 H.sub.7 OH                                                                     235.3   47.0  0.27   --                                    Diethyl ether                                                                           (C.sub.2 H.sub.25).sub.2 O                                                             193.6   36.3  0.27   --                                    Water     H.sub.2 O                                                                              374.1   218.3                                              ______________________________________                                         *data from Matheson Gas Data Book (1980) and CRC Handbook of Chemistry an     Physics (CRC Press, Boca Raton, Florida 1984).                                **T.sub.r = 1.03                                                         

In addition, a modifier may be added to the fluid, includingsupercritical fluids, to improve the solvent characteristics thereof.The most useful modifiers are the low to medium boiling point alcoholsand esters. Typical modifiers include methanol, ethanol, ethyl acetateand the like. The modifiers typically are added to the fluids atproportions of between about 0.1% and 20.0% by weight. The modifierscontemplated for use herein are most typically not supercritical fluidsat the disclosed operating conditions. Rather, the modifiers are simplydissolved in the fluid solvents, including the supercritical fluidsolvents, to improve their solvent properties.

In one embodiment the chosen enhancer is combined with a supercriticalfluid at the described proportions prior to feeding the supercriticalfluid to the extraction vessel. Alternatively, the supercritical fluidis fed to the extraction vessel without the enhancer. The enhancer isthen introduced into the extraction vessel and thereby combined with thesupercritical fluid.

Some chelating agents that may be useful for solubilizing metals insupercritical fluids include:

                  TABLE II                                                        ______________________________________                                        COMMONLY USED METAL CHELATING AGENTS                                          ______________________________________                                        O-Donating Chelating Reagents                                                 Cupferron                                                                     Chloroanillic acid and related reagents                                       β-diketones and related reagents                                         N-Benzoyl-N-phenylhydroxylamine and related                                   reagents                                                                      Macrocyclic compounds                                                         N-Donating Chelating Reagents                                                 α-dioximines                                                            Diaminobenzidine and related reagents                                         Porphyrines and related reagents                                              O,N-Donating Chelating Reagents                                               8-Hydroxyquinoline                                                            Nitrosonapthols and nitrosophenols                                            EDTA and other complexionates                                                 Diphenylcarbazide and diphenylcarbazone                                       Azoazoxy BN                                                                   S-Donating Chelating Reagents                                                 Sodium diethlydithiocarbamate and related                                     reagents                                                                      Dithizone and related reagents                                                Bismuthiol II                                                                 Thiothenoyltrifluoracetone                                                    Thioxine                                                                      P-Donating Chelating Reagents                                                 Tributylphoshpate and related reagents                                        ______________________________________                                    

B. specific Examples of Copper Extraction

The following examples illustrate several specific embodiments forpracticing the method of the present invention. These examples areprovided solely to illustrate the invention and should not be construedto limit the invention to those particular features shown.

EXAMPLE I

This example illustrates a process for bulk and continuous SFEextraction, with reference to the extraction device shown in FIG. 1. Thesample bulk extraction device of FIG. 1 includes a source 20 ofsupercritical CO₂ that directly extracts Cu ions from aqueous solution.The supercritical CO₂ was delivered from a high pressure syringe pump 22and passed through solid lithium bis(trifluoroethyl)dithiocarbamate(FDDC) which was contained in a stainless steel, high pressureextraction vessel 24. The supercritical CO₂ containing dissolved FDDCwas subsequently introduced to a second extraction vessel 26 fitted withquartz windows and containing an aqueous solution 28 of Cu(NO₃)₂ below aSCF CO₂ phase. Extraction of Cu was monitored by UV-Visible spectroscopyas the formation of the CO₂ soluble complex Cu(FDDC)₂, the structure ofwhich is shown below. ##STR13## For the bulk extraction system of FIG. 1where mixing of the aqueous and supercritical fluid phases were carriedout by simple stirring, extraction efficiency was found to be pressuredependent. As seen from FIG. 2, the initial extraction rate is veryrapid. Within about 5 minutes, approximately 70% of the complexsaturation value at the given fluid density was achieved. Finally,quantitative extraction of the copper ions had been achieved in about 30minutes with a pressure of 79.3 bar and a temperature of 35° C. At 50°C., quantitative extraction occurred at a CO₂ density of 0.29 g/cm₃ inless than 5 minutes, thus indicating an appropriate rate increase from35° C.

A continuous extraction process was set up using the apparatus of FIG. 1in which a high pressure recirculating pump was installed in line withthe extraction system. Quantitative extraction occurred even morerapidly than in the bulk system with quantitative extraction occurringin less than 5 minutes at 35° C.

The above results demonstrate that quantitative extraction of metal ionsfrom an aqueous or liquid matrix is possible with both bulk andcontinuous reactor systems using a dissolved molecular complexionate inthe supercritical fluid phase.

EXAMPLE II

This example illustrates a process for extracting metals and/ormetalloids from a solid matrix. The same apparatus used in example I wasalso used in the extraction of Cu ions adsorbed on a solid matrix. Inthis case, solid Cu(NO₃)₂ adsorbed on silica (SiO₂) was placed in thesecond extraction vessel. Supercritical CO₂ containing dissolved ligand,FDDC, was then introduced into the cell. Extraction efficiency was againmonitored spectroscopically as Cu(FDDC)₂ dissolved into thesupercritical CO₂. Initial extraction rates were again very rapid. Inabout 20 minutes the CO₂ phase was saturated with dissolved metalcomplex. In this case approximately 80% of the Cu ions could beextracted at a final fluid density of 0.55 g/cm³.

In both examples I and II, it was found that the metal chelate could becollected in whole by precipitation from the supercritical CO₂ bydecreasing the pressure of the system. It is also seen that the presentinvention is useful for removing contaminants from a complex matrix,which is either a solution or solid sample in which are present manydifferent species (including organic and inorganic species).

C. Fluorinated Celating Agents EXAMPLE III

This example describes the use of fluorinated chelating agents for SFEaccording to the present process. In developing the above extractionmethods, it was found that fluorination of complexing agents yieldedenhanced solubility behaviors of the metal chelates in supercriticalCO₂. Fluorination of sodium diethyldithiocarbamate (DDC) to form sodiumor lithium bis(trif luoroethyl)-dithiocarbamate (FDDC) was found toincrease the solubilities of the metal-diethyldithiocarbamates by almost3 orders of magnitude. For example, Cu(DDC)₂ has a UV-Visible determinedsolubility in supercritical CO₂ of (1.1±0.2)×10⁻⁶ mol/L. Upon thefluorination of the terminal methyl groups of DDC, the solubility ofCu(FDDC)₂ in supercritical CO₂ was determined to be (9.1±0.3)×10⁻⁴mol/L. Another example of increased solubility in supercritical CO₂ wasobtained with the β-diketone acetylyacetonate (ACAC). The solubility ofCu(acac)₂ in supercritical CO₂ was substantially increased by formingCu(HFACAC)₂, which is the hexaf luoroacetyl acetonate.

The present inventors have found that fluorination of chelating agentsfavors the extraction of fluorinated metal chelates in supercriticalCO₂. The behaviors of metal fluorinated diethyldithiocarbamate (FDDC)complexes in supercritical fluid chromatography (SFC) have notpreviously been reported. This example illustrates the separation ofarsenic DDC and FDDC complexes in SFC using CO₂ as a mobile phase.

A Lee Scientific Model 602 supercritical fluid chromatograph with aNeslab RTE-100 constant temperature bath was used for all analysisreported in this example. This system was equipped with a timed-splitrotary injection valve and an FID. All chromatograms were run usingsupercritical CO₂ as the mobile phase (Matheson) and a 5-meter 100-um IDby 195-um OD SB-Methyl-100 Superbond capillary column (Lee Scientific).The chromatographic signals were recorded and processed using a HP 3390Aintegrator. The temperature and density conditions for the analysis werecomputer controlled and are reported below.

The stock solutions (Zn, Ni, Co, Fe, Hg, As, Sb, and Bi) used in thisstudy were Baker Analyzed Reagents from the J. T. Baker ChemicalCompany. Sodium diethyldithiocarbamate (NaDDC) was purchased from theFisher Scientific Company. Other chemicals such as chloroform anddichloromethane were purchased from EN Science. Ammonium acetate bufferwas prepared by mixing 120 g of glacial acetic acid (J. T. BakerUltrapure Reagent) and 134 g of concentrated NH₄ OH (Aldrich A.C.S.Reagent) and diluting to 1 liter. The pH value was adjusted by drop wiseaddition of HNO₃ and/or NH₄ OH. Deionized water was prepared by passingdistilled water through an ion exchange column (Barnstead ultrapurewater purification cartridge) and a 0.2-um filter assembly (Pall Corp,Ultipor DFA). Sodium bis(trifluoroethyl)amine was purchased from PCRResearch Chemicals.

The metal-DDC and FDDC complexes were prepared by adding an excessamount of ligand to the metal solutions at the pH indicated in Anal.Chem 54:2536 (1982). The resulting precipitates were extracted intochloroform, and the organic phase was washed with deionized water afterphase separation. Purification of the metal complexes was done usingrecrystallization from a chloroform/ethanol solution (1:1 v/v). Otherchemicals used in the synthesis, including sodium amide, carbondisulfide, and potassium hydroxide were all obtained from AldrichChemical Company. All containers used in the experiments were acidwashed, rinsed several times with deionized water, and dried in a class100 clean hood.

The conditions used for chromatographic separation were an oventemperature of 100° C. with initial CO₂ pressure of 100 atm, followed bya 6.50 min hold time with a pressure ramp of 4.0 atm/min to a finalpressure of 200 atm. Sample injection time was 0.1 seconds, whichamounts to a calibrated 80 nL sample injection. Flame ionizationdetector (FID) temperature was 325° C. Under these conditions,separation and detection of some metal dithiocarbamate complexes such asAs(DDC)₃₁ Ni(DDC)₂, Pb(DDC)₂, and Zn(DDC)₂ were possible. However,judging from the uneven reproducibility of results and broad peakshapes, these particular metal complexes apparently have a lowersolubility in supercritical CO₂. These difficulties were furthercompounded by sample decomposition and retention within the columnresulting in chromatographic memory and subsequent column contamination.

Fluorination of the ligand changes the chromatographic behavior of thesemetal chelates. FIG. 3 illustrates this point with a comparison of asample analyzed by capillary SFC containing the same concentration(6×10⁻⁴ M) of As(FDDC)₃ and As(DDC)₃ with docasane (C₂₂ H₄₆) being usedin this case as an internal standard. The As(DDC)₃ peak is typical ofmetal-DDC complexes, being broader and less reproducible. On the otherhand, the As(FDDC)₃ peak is sharp and well-defined, with a shorterretention time relative to the corresponding DDC complex. Thechromatographic results of As(FDDC)₃ were reproducible without any ofthe column contamination problems that were encountered using DDC.

FIG. 4 shows a series of metal-FDDC complexes that were separated anddetected, which includes Zn, Ni, Co, Fe, Hg, As, Sb, and Bi. Peak (a) isNaFDDC, peak (b) is Zn(FDDC)₂, peak (c) is Ni(FDDC)₂, peak (d) isCo(FDDC)₃ peak (e) is Fe(FDDC)₃, peak (f) is Hg(FDDC)₂, peak (g) isAs(FDDC)₃ peak (h) is Sb(FDDC)₃, and peak (i) is Bi(FDDC)₃. The valenceof the metal ionic species is the same as the number of FDDC ligands inthe chelate. This chromatograph shows the ability of the present methodto separate and detect arsenic from a mixture of metal complexes. Thedetection limit of these metal chelates is generally in the order of 1ppm. The extraction procedure serves as a preconcentration step for SFCanalysis. With a preconcentration factor of 10 to 100, this techniquecan be used for trace analysis. The percentages of recovery of thesetrace metals using FDDC extraction are generally <95%.

EXAMPLE IV

This example describes the calculation of the stability constants forcertain chelates useful for the present invention. Fluorination ofdiethyldithiocarbamate has been shown to increase the stability of metalchelates. A numerical value for the enhancement of the stabilityconstant of the arsenic complex was estimated using a competitionexperiment where As³⁺ was added in a sub-stoichiometric amount to amixture of equal amounts of Na-DDC and Na-FDDC. In this experiment, theconcentration of As³⁺ in the aqueous phase was 2.1×10⁻³ M and theconcentrations of each ligand were 2.1×10⁻² M. After extraction, theorganic phase was analyzed by supercritical fluid chromatography (SFC)to determine the relative amounts of As(FDDC)₃. The relative stabilityconstants of the two arsenic chelates can be calculated from thefollowing equilibrium relations:

    As.sup.3+ +3 FDDC →As(FDDC).sub.3                   (3)

    As.sup.3+ +3 DDC →As(DDC).sub.3                     (2)

    K.sub.1 /K.sub.2 =([As(FDDC).sub.3 ]/[As(DDC).sub.3 ]) ([DDC]/[FDDC]).sup.3 (3)

Since [DDC]/[FDDC] equals 1 and K₂ has been determined by the inventors(7.1×10²³), from the relative concentrations of the two arseniccomplexes, K, can be calculated from equation (3). The value of K,determined from this experiment is 2.1×10²⁴.

EXAMPLE V

The effect of fluorination on the solubility of several metal chelatesis explored in this example. In accordance with one aspect of thepresent invention, the chelating agent is fluorinated to improve thesolubility of the metal chelate in the supercritical fluid and enhancemetal extraction.

The chelating agents in this example are DDC, FDDC, "H-crown","F2-crown", and "F6-crown." The structural formulae of these agents are:##STR14## "H-crown" refers to the non-fluorinated mnolecule, "F2-crown"has two added fluorine atomns, and "F6-crown" has six added fluorineatoms.

Table III shows increases in solubility at the lower pressure/densitiesof CO₂. The solvation power of CO₂ at the higher pressures/densitiesincreases significantly such that it is mnuch more like a regular liquidorganic solvent at those pressures. At such high pressures, thesolubility enhancement from fluorine diminishes. This is advantageousbecause the solubility increase from fluorination occurs at moderate,easily achievable conditions.

                  TABLE III                                                       ______________________________________                                        Solubilities of Fluorinated and Non-Fluorinated                               Metal Chelates in Supercritical CO.sub.2 at 100 atm and 50° C.         Metal Chelate                                                                             Solubility  Ratio(FDDC/DDC)                                       ______________________________________                                        Na(FDDC)    (4.7 ± 0.3) × 10.sup.-4                                                          3.1                                                   Na(DDC)     (1.5 ± 0.1) × 10.sup.-4                                  Cu(FDDC).sub.2                                                                            (9.1 ± 0.3) × 10.sup.-4                                                          830                                                   Cu(DDC).sub.2                                                                             (1.1 ± 0.2) × 10.sup.-6                                  Ni(FDDC).sub.2                                                                            (7.2 ± 1.0) × 10.sup.-4                                                          850                                                   Ni(DDC).sub.2                                                                             (8.5 ± 1.0) × 10.sup.-7                                  Co(FDDC).sub.3                                                                            (8.0 ± 0.6) × 10.sup.-4                                                          330                                                   Co(DDC).sub.3                                                                             (2.4 ± 0.4) × 10.sup.-6                                  Bi(FDDC).sub.3                                                                            (<10.sup.-7)                                                      Bi(DDC).sub.3                                                                             (1.3 ± 0.1) × 10.sup.-6                                  BiFDDC).sub.3.sup.a                                                                       (7.3 ± 1.0) × 10.sup.-4                                                          81                                                    Bi(DDC).sub.3.sup.a                                                                       (9.0 ± 0.6) × 10.sup.-6                                  ______________________________________                                         .sup.a Solubility calculated at 150 atm and 50° C., corresponding      to a CO.sub.2 density 0.66 g/cm.sup.3.                                   

Dithiocarbamate chelating agents are somewhat nonselective for the metalwith which a chelate is formed. Fluorination of a dithiocarbamatechelating agent, however has a greater effect on increasing thesolubility of transition metal chelates such as Cu, Ni, and Co than onalkali earth metals such as Na.

                                      TABLE IV                                    __________________________________________________________________________    Solubility comparison of the lariat crown                                     ethers in supercritical CO.sub.2 at 50° C. C                           Solubilities                                                                  Pressure/Density                                                                       F-6 crown F-2 crown H crown                                          __________________________________________________________________________    100 atm/0.388                                                                          (3.0 ± 0.2)10.sup.-5 [M]                                                             (7.9 ± 0.5)10.sup.-8 [M]                                                             (7.6 ± 0.7)10.sup.-8 [M]                      117 atm/0.528                                                                          (9.0 ± 0.4)10.sup.-5 [M]                                                             (8.4 ± 0.5).sup.-5 [M]                                                               (2.2 ± 0.2)10.sup.-5 [M]                      150 atm/0.662                                                                          (1.3 ± 0.1)10.sup.-4 [M]                                                             (9.9 ± 0.5)10.sup.-5 [M]                                                             (7.6 ± 0.7)10.sup.-5 [M]                      200 atm/0.767                                                                          (1.6 ± 0.1)10.sup.-4 [M]                                                             (1.2 ± 0.1)10.sup.-4 [M]                                                             (1.2 ± 0.1)10.sup.-4 [M]                      250 atm/0.833                                                                          (1.8 ± 0.1)10.sup.-4 [M]                                                             (1.5 ± 0.1)10.sup.-4 [M]                                                             (1.8 ± 0.2)10.sup.-4 [M]                      300 atm/0.883                                                                          (2.1 ± 0.1)10.sup.-4 [M]                                                             (2.1 ± 0.1)10.sup.-4 [M]                                                             (2.1 ± 0.2)10.sup.-4 [M]                      350 atm/0.922                                                                          (2.4 ± 0.2)10.sup.-4 [M]                                                             (2.3 ± 0.1)10.sup.-4 [M]                                                             (2.7 ± 0.3)10.sup.-4 [M]                      400 atm/0.955                                                                          (2.9 ± 0.2)10.sup.-4 ]M]                                                             (2.5 ± 0.2)10.sup.-4 [M]                                                             (3.3 ± 0.3)10.sup.-4 [M]                      __________________________________________________________________________     % weight of Fluorine in MetalFDDC Complexes is approximately 40%         

These solubility characteristics are shown in FIG. 5.

D. Chelate Selectivity EXAMPLE VI

The crown ethers are generally known as macrocyclic polyethers. Manyrelated compounds have been made involving heterocyclic rings containing12 to 60 atoms. Crown ethers are particularly useful as chelating agentsbecause they can be made selective for particular ligands. There are,for example, optimum polyether ring sizes for different alkali metalcations. A 15 to 18 member ring has an optimal cavity size for chelatinga cation having the radius of sodium; an 18 member ring is optimal forchelating potassium; and an 18 to 21 member ring is most suitable forcesium.

Functional groups on the crown ether also play a role in complexingcations. Some crown ethers with pendant carboxylate functional groups(such as sym-dibenzo-16-crown-5-oxyacetic acid) are quite efficient andselective for extracting trivalent lanthanide ions. The negativelycharged carboxylate group is believed to attract the positively chargedlanthanide ions, which are then inserted into a ring having theappropriate cavity size. For lanthanides and actinides, the cavity sizeis preferably a 16 member crown ether ring. The extraction does notrequire specific counter anions and is reversible with respect to pH.Lanthanides complexed with the lariat crown ether in an organic solventcan be stripped with a dilute acid to regenerate the chelating agent.

Spectroscopic evidence indicates that both the crown cavity andcarboxylate group are involved in the complexation with a lanthanide toligand ratio of 1:2, suggesting a possible sandwich formation.Experimental results also show that the efficiency of extraction oftrivalent lanthanide ions is much greater (>10²) than that of alkalimetal ions such as Na+, although the ionic radius of Na+ (0.96 Å) issimilar to those of the lanthanide ions (1.15 Åfor La⁺³). The reducedefficiency for Na+ has been attributed to the high degree of ionizationof Na+ with the carboxylate group of the macrocycle. The crown ethercarboxylic acid also shows lower extraction efficiencies for thealkaline earth metal ions (Ca²⁺, Sr²⁺, and Ba²⁺). Thus, thefunctionalized macrocycles apparently act as bi-functional chelatingagents which, with proper design, can be made more selective than theconventional neutral crown ethers.

The lariat crown ether system also shows high extraction efficiency forLu³⁺ (ionic radius 0.93 Å), and even has a selectivity for Lu³⁺ overLa³⁺ by as much as an order of magnitude depending on the solvent. Theobserved selectivity is believed to result from small differences inionic radius and bonding of the lanthanides with the ligand. Using thistype of crown ether carboxylic acid, extraction of uranium and rareearth elements in seawater (with concentrations in the order of partsper billion or less) can be achieved quantitatively.

E. Hydroxamic Acid Crown Etters EXAMPLE VII

The crown ethers of the present invention include many hydroxamic acidsthat are described by the following empirical formula: ##STR15## whereinn is an integer of from 0 to 6, X is a dibenzo crown ether of theformula dibenzo-[13+3m]-crown-[4+m]-ether wherein m is an integer offrom 0 to about 5 or so and R₈ is a lipophilic group which impartslipophilicity to the hydroxamic acid derivative. The size of the dibenzocrown ether may be varied provided the metal to be extracted fits in thering such that the donor atoms coordinate to the metal (or to waterassociated with the metal). However, crown ethers in which the ring sizeof the crown is too large for a satisfactory host/guest interaction arenot suitable. Some preferred dibenzo crown ethers are those in which mis 0, 1 or 2, and are, respectively, dibenzo-13-crown-4 ether,dibenzo-16-crown-5 ether, and dibenzo-19-crown-6 ether.

Since the chelating agents of the present invention are useful for theextraction of metal ions of the lanthanide and actinide series and ofyttrium and scandium from aqueous medium, the hydroxamic acidderivatives of this invention are preferably lipophilic in order tominimize or even prevent the chelating agent from partitioning in theaqueous phase. Generally, the greater the lipophilicity of the chelatingagent, the better the chelating agent will perform. Although R may behydrogen, it is preferably a lipophilic moiety. Thus, R is preferably ahydrocarbyl group having from 1 to about 18 carbon atoms and is selectedfrom the group consisting of alkyl, cycloalkyl, alkenyl and aryl groups.These groups may also be substituted with other functional groups. Forexample, if aryl is phenyl, the phenyl may be substituted with electronwithdrawing groups such as fluorine, or it may be substituted withelectron donating groups such as methoxy. By way of illustration and notin limitation, the phenyl group may be completely substituted withfluorine, such that R is C₆ F₅ or it may be a 3,5-di-trifluoromethylphenyl group.

Fluorinated derivatives of hydroxamic acid are believed to be useful inthe extraction of lanthanide and actinide metal ions using supercriticalcarbon dioxide. The solubilities of hydroxamic acid chelates isrelatively low in supercritical CO₂. Hence, side chain lipophilicity andfluorination are preferred to increase solubility of the chelate insupercritical CO₂.

It is believed that R groups of from about 6 to about 10 carbon atomswill sufficiently increase lipophilicity and maximize solubility of thechelate in supercritical CO₂. It is also believed that higher extractionefficiency is achieved with R groups of greater lipophilicity, that is,where R is a longer chain hydrocarbon, and that hydroxamic acids inwhich the R group is aryl are often more selective. The lipophilicity ofthe side chain should be less important for a more polar supercriticalfluid such as N₂ O.

In another aspect, the dibenzo ether derivatives of hydroxamic acid ofthe present invention are described by the following empirical formula:##STR16## wherein Y is a member selected from the group consisting of(CH₂)₃,[[CH₂ CH₂ O]_(N) CH₂ CH₂ ] wherein n is an integer of from 1 toabout 4, and CH₂ CONHCH₂ CH₂ NHCOCH₂ ; and R₉ and R₁₀, which may be thesame or different, are selected from the group consisting of H and ahydrocarbyl group having from 1 to about 18 carbon atoms, which isselected from the group consisting of alkyl, cycloalkyl, alkenyl andaryl groups. As with the hydroxamic acid derivatives represented byformula (I), the R groups are preferably lipophilic when thesupercritical fluid is CO₂. Further, as with the hydroxamic acidderivatives represented by formula (I) above, these groups may besubstituted with other functional groups. Fluorination of these groupswould also be preferred when the supercritical fluid is relativelynon-polar (such as supercritical carbon dioxide).

In yet another aspect, the present invention provides a bis-dibenzocrown ether derivative of a hydroxamic acid represented by the followingempirical formula: ##STR17## wherein X is a dibenzo crown ether of theformula dibenzo-[13+3m]-crown-[4+m]-ether and m is an integer of from 0to about 5 or so, and R₁ l is hydrogen or a lipophilic hydrocarbyl grouphaving from 1 to about 18 carbon atoms and is selected from the groupconsisting of alkyl, cycloalkyl, alkenyl and aryl groups. The R groupsmay be substituted with other functional groups, preferably fluorine.The dibenzo crown ether X may be the same or different and its size maybe varied provided the metal to be extracted fits into the ring topermit chelation. The preferred dibenzo crown ethers are those in whichm is 0, 1 or 2, and are, respectively, dibenzo-13-crown-4 ether,dibenzo-16-crown-5 ether and dibenzo-19-crown-6 ether. Fluorination ofthe benzene rings is especially preferred when the crown ether is to beused with a relatively non-polar supercritical fluid such as CO₂.

The present invention further provides a tridentate derivative of ahydroxamic acid represented by the formula: ##STR18## where R₁₂ or R₁₃may be H, but preferably R₁₂ or R₁₃ is a lipophilic moiety having offrom 1 to about 18 carbon atoms and is selected from the groupconsisting of alkyl, cycloalkyl, alkenyl and aryl groups. The R₁₂, R₁₃groups may be the same or different moieties, and may be substitutedwith other functional groups, such as fluorine.

Details of the synthesis of specific crown ethers useful in thisinvention are set forth in co-pending U.S. Patent application Ser. No.07/714,265, filed June 12, 1991, which has been incorporated byreference.

E. Specific Waste-Treatment Embodiment EXAMPLE VIII

One proposed embodiment for a continuous selective-chelationsupercritical fluid extraction process is illustrated in FIG. 6. Thisprocess is suitable for chelating metals that are contained in solid orliquid waste held in a container 50. A supercritical fluid, such ascarbon dioxide gas, is supplied from a CO₂ gas reservoir 52 which isconnected by a conduit 54 containing a valve 55 to a pressurization unit56 that increases the pressure on the gas to greater than 73 atmospheresat a temperature greater than 32° C. to form supercritical carbondioxide. The supercritical CO₂ then travels through a valve 57 andconduit 58 to a reservoir 60 that holds a solid or liquid chelatingagent, such as any of the agents described in the earlier examples ofthis specification. The CO₂ is there passed through a column containingsolid or liquid chelating reagent to extract the chelating agent intothe supercritical fluid CO₂ stream. The supercritical fluid andchelating agent leave reservoir 60 through a conduit 62 and areintroduced into container 50.

The supercritical fluid/chelating agent is intimately mixed with thesolid or liquid waste in container 50 using either a batch or continuousprocess. In a batch process, simple mixing would occur through stirringor sonification. Alternatively, mixing could occur by allowing CO₂ toflow through a column of solid waste. In a continuous mixing embodiment,CO₂ would flow through a column of solid waste material. Continuousmixing with a liquid waste could be achieved with counter current flow.

After mixing, the metal chelate and CO₂ is removed through a conduit 64.A depressurizer valve 66 is present in line 64 to reduce the pressure tobelow seventy-two atmospheres such that the metal chelate precipitatesin container 67. The CO₂ gas is then recycled by pump 68 through line 70to gas reservoir 52. Metal chelates can be removed from the bottom ofcontainer 67 through line 72 such that the chelating agent can beregenerated from the metal chelate. When regeneration of the chelatingagent is desired, metal ions can be stripped from the chelate using anitric acid solution having a pH less than one.

The extraction system should be thermally controlled, either by knownelectrical means or immersion in a constant temperature bath. Thermalcontrol allows the carbon dioxide or other supercritical fluid to bemaintained above its supercritical temperature.

F. Extraction of Metal Ions with β-Diketones and/or Trialkyl Phosphatesand Trialkylphosphine Oxides

Trivalent metal ions can be extracted by fluid CO₂, particularlysupercritical CO₂, containing β-diketones. Carbon atoms of a ketone areassigned greek letters to designate their position relative to thecarbonyl carbon. The first carbon adjacent the carbonyl carbon isdesignated α, the second such carbon being designated β, and so on. Aβ-diketone has at least two ketone carbonyls wherein one ketone carbonylis located on a carbon β to the other ketone functionality. Theextraction efficiency appears to be enhanced when the β-diketone ishalogenated, particularly when the β-diketone is fluorinated.

A number of fluorinated β-diketones are commercially available from suchcompanies as Aldrich Chemical Company of Milwaukee, Wisc. Theseβ-diketones form stable complexes with lanthanides and actinides, andhence are useful ligands for SFE of the f-block elements. Extraction andseparation of actinides by supercritical fluids are of particularinterest because of the potential applications for nuclear wasteanalysis and management.

Trialkyl phosphates, such as tributyl phosphate, also function well asligands for the extraction of metal and metalloids from liquids andsolids. This is particularly true for acidic aqueous systems. Mixedligands, such as β-diketones and trialkyl phosphates, also have beenfound to be useful for the supercritical fluid extraction of metals andmetalloids, particularly the actinides, using CO₂. Using a mixture ofligands comprising β-diketones and trialkyl phosphates appears toprovide a synergistic extraction capability.

1. Halogenated β-diketones

Several fluorinated β-diketones have been used for the extraction ofuranyl and Th(IV) ions using carbon dioxide fluid solvents as shownbelow in Table V. Except TTA, all other β-diketones tested are liquidsat room temperature and at atmospheric pressure.

                  TABLE V                                                         ______________________________________                                        Fluorinated β-diketones Used for The Extraction of                       Uranyl and Th(IV) Ions Using Supercritical Carbon Dioxide                     β-Diketone Abbr.                                                                      R.sub.1  R.sub.2                                                                              Mol. Wt. B.P.                                                                          ° C.                             ______________________________________                                        Acetylacetone                                                                           AA     CH.sub.3 CH.sub.3                                                                           100.12   139°                           (760                                                                          Torr)                                                                         Trifluoro-                                                                              TAA    CH.sub.3 CF.sub.3                                                                           154.09   107°                           acetylacetone                                                                 Hexafluoro-                                                                             HFA    CF.sub.3 CF.sub.3                                                                           208.06   70-71°                         acetylacetone                                                                 Thenoyltri-                                                                             TTA    Thenoyl  CF.sub.3                                                                           222.18   103-104°                       fluoroacetone                                                                 (9 Torr)                                                                      Heptafluoro-                                                                            FOD    C(CH.sub.3).sub.3                                                                      C.sub.3 F.sub.7                                                                    296.18    33°                           butanoyl-                                                                     pivaroylmethane                                                               (2.7 Torr)                                                                    ______________________________________                                         The fluorinated betadiketones were purchased from the Aldrich Chemical        Company of Milwaukee, WI, and were used without further purification.    

β-diketones exist in at least two tautomeric forms, the "keto" tautomerand the "enol" tautomer. Tautomerism is a type of isomerism in whichmigration of a hydrogen atom results in two or more structures calledtautomers. β-diketones react with metal ions to form chelates eitherthrough the enol tautomer or through an enolate anion (a negativelycharged "enol" form of the molecule) illustrated by the followingequilibria: ##STR19##

The presence of a small amount of water has been found to significantlyincrease the CO₂ extraction efficiency of metal or metalloid ions from asolid support using fluorinated β-diketones as an extractant. Withoutlimiting the invention to one theory of operation, water moleculeslikely form adducts with the uranyl/β-diketone complex. This mayfacilitate the release of the complex from the active sites of thecellulose-based matrix. Therefore, all solid extraction experiments inthis study were conducted under wet conditions. One skilled in the artwill realize that the amount of water used during the extraction processmay vary. However, where about 10 μg of metal ions are to be extracted,it has been determined that at least about 10 μL of water should beadded to the system prior to the extraction process. In other words, itappears sufficient to use about 1 μg of water per 1 μg of metal ion.

The following examples describe the extraction of metal and metalloidions from various media, including both solid and liquid media, usingfluorinated β-diketones. These examples are provided solely toillustrate certain preferred embodiments of the invention, and shouldnot be construed to limit the invention to the particular embodimentsdescribed. For instance, the examples illustrate the use of fluorinatedligands, but the invention should not be limited to just fluorinatedcompounds as other halogenated ligands also may perform satisfactorily.

EXAMPLE IX

This example describes the extraction of uranyl and Th(IV) ions from asolid support using only supercritical CO₂. The solid support used inthis example was a cellulose filter paper obtained from Whatman Ltd.(Maidstone, England). The procedure for extracting the metal ions wassubstantially as described above in example II. Uranyl (UO₂)²⁺ and Th⁴⁺solutions were prepared from their nitrate salts, which were obtainedfrom Baker Chemical Co. and from Mallinckrodt, Inc. (St. Louis, Mo.),respectively. All other chemicals used were analytical-reagent grade.

Solid samples were prepared by spiking 10 μg each of a mixture of(UO₂)²⁺ and Th⁴⁺ on pre-washed filter papers (Whatman 42, 0.5 cm×2 cm insize). The filter papers were washed with Ultrex HNO₃ and rinsed withdeionized water. The spiked filter papers were allowed to air dry at aroom temperature of about 23° C. All extractions were conducted with theSFE extraction apparatus described above in example I or a commercialextraction vessel that is available from Dionex, of Sunnyvale Ca.

For solid sample extraction, a glass tube (0.5 cm. i.d. and 3 cm. inlength) was plugged at one end with a piece of glass wool that wascleaned with Ultrex nitric acid. A spiked sample was added to the openend of the glass tube. Water (10 μl) and about 80 μmole of a ligand wereintroduced to the sample in that order. The open end of the tube wasthen plugged with clean glass wool, and the sample tube was then placedin the extraction vessel and installed in an SFE oven. To extract themetal ions from the solid support, the solid samples were subjected to10 minutes of static extraction followed by 10 minutes of dynamicflushing at 60° C. and 150 atm. These conditions were found satisfactoryfor the extraction of uranyl ions from the cellulose based filter paperand from sand by supercritical CO₂.

In order to determine the amount of metal ions extracted using theprocess, all samples and standards were irradiated for 2 hours in a 1 MWTRIGAR nuclear reactor at a steady flux of 6×10¹² n cm⁻² s⁻¹. Afterirradiation, the samples were cooled for 24 hours before counting. Eachsample was counted for 100 s in a large-volume ORTEC Ge(Li) detectorwith a resolution (FWHM) of about 2.3 keV at the 1332-keV⁶⁰ Co peak.Uranium was determined by the 228 keV gamma emitted during the decay ofits daughter ³³⁹ Np (2.36 d). Neutron activation of ²³² Th produces ashort lived radioisotope ²³³ Th with a half life of 22.2 minutes. Itsdaughter product, ²³³ Pa (27 days) further decays to ²³³ U with theemission of 311 KeV gamma which was used for quantitative determinationof thorium.

The results for the extraction of uranyl and Th(IV) ions from thecellulose based filter papers are provided below in Table VI. It isimportant to note from Table VI that free uranyl and Th(IV) ions cannotbe extracted (<2%) by supercritical CO₂ in the absence of a ligand.

EXAMPLE X

This example concerns the extraction of uranyl and Th(IV) ions from asolid cellulose support using a β-diketone. In this specific example,acetylacetone (AA) was used as the β-diketone, and the extractionprocedure used was substantially as described above in example IX. TableVI shows that the extraction of metal and/or metalloid ions from a solidsupport using a non-fluorinated β-diketone provides only limitedextraction capability. More specifically, AA extracted only about 10percent of the uranyl(VI) ions from the solid support, and only about 12percent of the Th(IV) ions from the support.

EXAMPLE XI

This example describes the extraction of uranyl and Th(IV) ions from asolid cellulose support using FOD (heptafluorobutanoylpivaroylmethane),a fluorinated β-diketone. The extraction procedure used in this examplewas substantially as described above in example IX. FOD extracted about51 percent of the uranyl(VI) ions, and about 80 percent of the Th(IV)ions, from the solid support (Table VI). By comparing the results fromthe extraction of metal ions using AA, it can be seen that halogenatedβ-diketones, particularly the fluorinated β-diketones, provide anenhanced capability for extracting metal ions from a solid support usingCO₂ SFE.

EXAMPLE XII

This example describes the extraction of uranyl and Th(IV) ions from asolid cellulose support using TTA (thenoyltrifluoroacetone). Theextraction procedure used in this example was substantially as describedabove in example IX. TTA was able to extract about 70 percent of theuranyl(VI) ions, and about 82 percent of the Th(IV) ions. TTA providedthe best extraction of metal ions from a solid support using CO₂ SFE.

In a manner similar to that described above in examples XI-XII,Uranyl(VI) and Th(IV) ions have been successfully extracted from solidmaterials using CO₂ SFE. The results of these extractions are presentedbelow in Table VI.

                  TABLE VI                                                        ______________________________________                                        Extraction of Uranyl and Th(IV) Ions                                          From a Solid Cellulose Support                                                        Ligand                                                                        Amount     Percent Extraction (%)                                     β-Diketone                                                                         (μmole)   U(VI)    Th(IV)                                        CO.sub.2                                                                      ______________________________________                                        None       0           <2      >1                                             FOD       80           51 ± 3                                                                             80 ± 3                                      TTA       80           70 ± 3                                                                             82 ± 4                                      HFA       80           40 ± 2                                                                             69 ± 3                                      TAA       80           15 ± 2                                                                             22 ± 3                                      AA        80           10 ± 2                                                                             12 ± 2                                      ______________________________________                                         *Each solid cellulose sample was 1 cm.sup.2 in area and contained 10 μ     of U and 10 μg of Th.                                                 

From the preceding examples it can be seen that in the presence of thenon-fluorinated β-diketone AA, only about 10% of the spiked uranyl ionscan be removed by neat CO₂ at 60° C. and 150 atm. However, with afluorinated β-diketone present in the fluid phase, extraction of thespiked uranyl ions in the filter paper by supercritical CO₂ issignificantly improved. The percent extraction of the spiked uranyl ionsby supercritical CO₂ varies from about 15% for TAA to about 70% for TTA.The percent extraction of Th(IV) by supercritical CO₂ from the cellulosebased matrix varies from about 22% for TAA to about 82% for TTA at 60°C. and 150 atm. The extraction efficiencies of the fluorinatedβ-diketones in supercritical CO₂ for Th(IV) ions are higher than thoseobserved for uranyl ions. TTA and FOD appear to be more effective thanTAA and HFA for the extraction of the actinide ions by neatsupercritical CO₂.

EXAMPLE XIII

This example illustrates how modifying the polarity of the fluid phasesignificantly increases the extraction efficiencies of metal chelates inCO₂, particularly supercritical CO₂. The procedure employed for thisexample was substantially as described above for examples X-XII, exceptthat a 5% methanol modified CO₂ was used as the extraction fluid. Thismixed solvent was prepared before the metal ions were exposed to thesolvent.

With the 5% methanol-supercritical CO₂, about 95-98% of the spikeduranyl ions were extracted from solid cellulose supports (Table VII).This is true when any one of the fluorinated β-diketones shown in TableVI is present in the fluid phase. The non-fluorinated ligand, AA, stillshows a lower extraction efficiency (45%) relative to the fluorinatedβ-diketones in methanol-modified CO₂ under the same conditions. Thisalso is true for the extraction of Th(IV) by methanol-modified CO₂ usingfluorinated β-diketones as extractants. Acetylacetone (AA) in methanolmodified CO₂ can extract Th(IV) up to 58% whereas the fluorinatedβ-diketones can extract Th(IV) in the 91-97% range under the sameconditions.

                  TABLE VII                                                       ______________________________________                                        Methanol Modified CO.sub.2 Extraction Fluid For Extracting                    Ions From A Solid Cellulose Support                                           CO.sub.2 + 5% Methanol                                                               Amount     Percent Extraction (%)                                      Ligand   (μmole)   U        Th                                             ______________________________________                                        FOD      80           98 ± 3                                                                              97 ± 3                                      TTA      80           96 ± 3                                                                              91 ± 3                                      HFA      80           95 ± 3                                                                              92 ± 4                                      TAA      80           98 ± 3                                                                              95 ± 3                                      AA       80           45 ± 2                                                                              58 ± 3                                      ______________________________________                                         *Each solid cellulose sample was 1 cm.sup.2 in area and contained 10 μ     of U and 10 μg of Th.                                                 

EXAMPLE XIV

This example describes the extraction of uranyl and Th(IV) ions fromaqueous samples. The procedure described in example I was used in thisexample as well. Water samples were prepared from a 0.1 M LiClO₄solution containing 2.5 μg/mL each of (UO₂)²⁺ and Th⁴⁺ at a pH of 3.5controlled by an acetate buffer (HAc/LiAc). Mine water samples werecollected from an open pit uranium mine near Spokane, Wash. For theextraction of ions from water, 4 mL of the spiked water sample wereplaced in the liquid extraction vessel. The pH of the solution wascontrolled by an acetate buffer. About 50-100 mg of TTA were loaded inthe ligand cylinder and placed upstream from the liquid extractionvessel. The samples were extracted dynamically at 60° C. and 150 atm for20 minutes. The extraction conditions for real mine waters were slightlymodified as specified in Table VIII.

When the extraction was completed, the sample was removed from theextraction vessel and analyzed by NAA (non-destructive neutronactivation analysis, which is a technique known to those skilled in theart). A standard solution containing 2.5 μg/mL each of thorium anduranium was irradiated and counted with the sample under identicalconditions. The extraction efficiencies were calculated based on theamounts of thorium and uranium found in the aqueous solution before andafter the extraction. The extracted uranyl and Th(IV) complexes in thefluid phase were collected in a glass vial containing 5 mL ofchloroform. The solutes trapped in the chloroform solution weredetermined by back-extraction with 50% HNO3 followed by NAA of the acidsolution. The results were also used for recovery calculations.

For the extraction of uranyl and Th(IV) ions from water, TTA was chosenas the extractant simply because it is a solid which is easier to handleexperimentally than the other β-diketones. This does not mean that theother ligands do not work for the extraction of metal ions from liquidsolutions. The extraction was performed dynamically at 150 atm and 60°C. for 20 minutes. In the absence of a ligand, free uranyl and Th(IV)ions cannot be extracted (<2%) by CO₂, or even supercritical fluid CO₂.This is true even with 5% methanol in the fluid phase. However, with TTApresent in the CO₂, the extraction efficiencies for the uranyl andTh(IV) ions are 38% and 70%, respectively (Table VIII).

Table VIII also shows the extraction results using tributyl phosphate(TBP). The empirical formula for TBP is shown below. ##STR20## TBP aloneshows low efficiencies (5-6%) for the extraction of uranyl and Th(IV)ions from the aqueous samples.

Table VIII also provides data for extractions using a mixed ligandcomprising TBP and TTA. This mixed ligand system is described in moredetail below in Section 2, "Synergistic Extraction Using Halogenatedβ-Diketones and Trialkyl

                  TABLE VIII                                                      ______________________________________                                        Extraction of U(VI) and Th(IV) Ions from Aqueous Solution                                  Percent Extraction (%)                                           Ligand         U        Th                                                    CO.sub.2                                                                      ______________________________________                                        TTA            38 ± 4                                                                              70 ± 5                                             TBP             5 ± 2                                                                               6 ± 2                                             TBP + TTA      70 ± 5                                                                              87 ± 5                                             ______________________________________                                         *Each water sample was about 4 ml and contained about 2.5 μg of Th at      pH of about 3.5, which was controlled by using an acetate buffer.        

EXAMPLE XV

This example illustrates the benefits obtained in terms of extractioncapability when a methanol modifier was used to extract uranyl(VI) andTh(IV) ions from an aqueous solution. In this particular example, TTAwas used as the ligand. The procedure used was substantially asdescribed above for example III.

Table IX shows that the addition of methanol substantially increases theefficiency of the extraction. More specifically, the addition of themethanol solvent modifier increased the extraction efficiency of uranyl(VI) ion from about 38% to 85 percent, and from about 70 percent toabout 90 percent for Th(IV) ions. Although the solubility of water insupercritical CO₂ is only on the order of about (0.1%, there was aconcern that the presence of 5% methanol would increase the solubilityof water in the fluid phase. However, the amount of water transported bythe methanol-modified fluid phase is estimated to be small, is on theorder of about one percent. This estimate was based on the fact that noseparate aqueous phase was observed in the collection vial containingchloroform which is known to have a solubility of about 1% for water at20° C.

                  TABLE IX                                                        ______________________________________                                        Percent Extraction of U(VI) and Th(IV) from an Aqueous                        Sample Using Methanol-Modified CO.sub.2                                       CO.sub.2 + 5% MeOH                                                                         U(VI) Th(IV)                                                     ______________________________________                                        TTA            85 ± 5                                                                             90 ± 5                                              ______________________________________                                         *Each water sample was about 4 ml and contained about 2.5 μg of Th at      pH of about 3.5, which was controlled by using an acetate buffer.        

EXAMPLE XVI

This example describes the extraction of metal ions from a solid sample.Solid samples were prepared by spiking 10 μg each of a mixture of(UO₂)²⁺ and Th⁴⁺ on sand. The spiked sand was allowed to air dry at aroom temperature of 23° C.

For this extraction, a commercial extraction vessel (Dionex, Sunnyvale,Ca.) having a volume of 3.5 mL was used. The experimental conditions forthe solid samples of this example were set at 10 minutes of staticextraction followed by 10 minutes of dynamic flushing at 60° C. and 150atm. These conditions were found satisfactory for the extraction ofuranyl ions from sand by supercritical CO₂.

TTA at an initial concentration of about 80 μmoles has been used as theligand for extracting U(VI) ions and Th(IV) ions from sand samples. TTAextracted about 72 percent of U(VI) ions from sand, and about 74 percentof the Th(IV) ions present in the sand sample, as shown in Table XI.

2. Synergistic Extraction Using Halogenated β-Diketones and Trialkylphosphates and/or Trialkylphosphine Oxides

The extraction of metal ions from various substrates was described aboveusing halogenated β-diketones, particularly fluorinated β-diketones, andtrialkyl phosphates as the ligands for such extractions. Other ligandsalso have been used for such extractions, as well as mixed ligandcompositions. This section describes the extraction of metal ions usingsupercritical CO₂ and a mixed ligand composition comprising β-diketonesand trialkyl phosphates.

EXAMPLE XVII

This example illustrates the extraction of metal ions using 80 μmoles oftributyl phosphate. For this extraction procedure, a Whatman filterpaper was spiked with 10 μgs of either U(VI) or Th(IV) ions. Theextraction procedure was substantially as described above for examplesII and IX.

Table X below shows the results for extracting uranyl(VI) ions andTh(IV) ions from a cellulose solid matrix using tributyl phosphate (TBP)as the sole extractant in supercritical CO₂. Although TBP can extracturanyl and thorium ions in supercritical CO₂, the extraction is notefficient. For instance, SFE using TBP and CO₂ can only extract about18% of the uranyl ions from the solid support, and only about 8% of theTh(IV) ions. Since TBP is a neutral ligand, anions such as nitrate,acetate, or perchlorate are probably involved in the transport of theuranyl-TBP complex in the fluid phase. These anions are present in thesystem since the spiked solutions were prepared with uranyl and thoriumnitrate in a LiClO₄ solution with pH controlled by an acetate buffer.

EXAMPLE XVIII

This example illustrates the extraction of metal ions using a mixedligand comprising a trialkyl phosphate, such as tributyl phosphate, anda β-diketone. For this extraction procedure, a Whatman filter paper wasspiked with 10 μgs of either U(VI) or Th(IV) ions. After that, 40 μmolesof TBP and 40 μmoles of a fluorinated β-diketone were added to thesample in this order. The procedure used for this example wassubstantially as described above for examples IX and XVII. Theβ-diketones selected for this example included FOD, TTA, HFA, TAA andAA. Equimolar amounts of the two ligands have been found to provideenhanced extraction capability. For instance, when TTA was used incombination with TBP as the extractant, an equal molar amount, such asabout 40 μmol, of each component was used to perform the extraction.However, this does not mean that only equimolar amounts of the ligandare suitable for forming the mixed ligand systems of the presentinvention. Currently, it is believed that from about 25 mole percent toabout 75 mole percent, preferably about 50 mole percent, of the trialkylphosphate or trialkylphosphine oxide can be used in combination with theβ-diketone to provide a useful and efficient mixed-ligand system.

The results of extractions with mixed ligands are shown below in TableX.

                  TABLE X                                                         ______________________________________                                        Extraction of U(VI) and Th(IV) from a Solid Cellulose                         Support Using CO.sub.2 SFE, TBP and β-Diketones                                  Ligand       Percent Extraction (%)                                   Ligand    Amount (μmole)                                                                            U        Th                                          ______________________________________                                        TBP       80             18 ± 3                                                                               8 ± 2                                   TBP + FOD 40 + 40        98 ± 4                                                                              95 ± 4                                   TBP + TTA 40 + 40        94 ± 4                                                                              92 ± 4                                   TBP + HFA 40 + 40        98 ± 5                                                                              98 ± 4                                   TBP + TAA 40 + 40        80 ± 4                                                                              70 ± 4                                   TBP + AA  40 + 40        57 ± 3                                                                              30 ± 3                                   ______________________________________                                         *Each solid cellulose sample (1 cm.sup.2 in area) contains 10 μg of U      and 10 μg of Th.                                                      

An important observation from Table X is that when TBP is mixed with afluorinated β-diketone the extraction efficiency for uranyl ions isstrikingly increased relative to those of the individual ligands. Forinstance, the extraction efficiency of TBP+FOD for uranyl ions is up toabout 98% compared with about 18% and 51% for individual TBP and FOD,respectively. This also is true for the TBP+TTA and the TBP+HFA mixedligands. The extraction efficiency for uranyl ions is 94% for the formerand 96% for the latter. The extraction efficiencies for Th(IV) bysupercritical CO₂ containing TBP and one of the three β-diketones (FOD,TTA, and HFA) are also high, in the range of 92-98%. Only TAA shows alower synergistic effect with TBP for the extraction of uranyl andTh(IV) ions relative to the other fluorinated β-diketones given in TableX. The extraction efficiencies for the actinides by neat supercriticalCO₂ containing the mixed ligands are comparable to those observed forthe methanol-modified CO₂ containing the fluorinated β-diketones as theextractant. The synergistic approach has the advantage of avoiding theuse of an organic solvent, such as methanol, in the SFE process.

The synergistic extraction of the actinide ions by TBP and TTA also wasobserved for the liquid samples (Table VIII). The extractionefficiencies for the uranyl and Th(IV) ions were raised to 70% and 87%,respectively, using the mixed TBP+TTA ligands in supercritical CO₂.Using 5% methanol modified CO₂ as the fluid, the extraction efficienciesfor the uranyl and Th(IV) ions were 85% and 90%, respectively, at 60° C.and 150 atm after 20 minutes of dynamic extraction with TTA as anextractant.

EXAMPLE XIX

This example concerns the extraction of Uranyl and Th(IV) ions from sandusing the synergistic extraction procedure discussed above in exampleXVIII. The procedure used for this extraction was substantially asdescribed above in examples IX and XVI. Table XI below illustrates theresults of such extractions with neat supercritical CO₂ containing TTA,TBP, and mixed TTA+TBP at 60° C. and 150 atm. Positive synergisticextractions of uranyl and Th(IV) ions also were found in this system forthe mixed ligands. The extraction efficiency for uranyl ions withTTA+TBP is 94% compared with 72% for TTA and 15% for TBP, individually.A similar synergistic effect is also observed for the extraction ofTh(IV) from sand.

                  TABLE XI                                                        ______________________________________                                        Extraction of U(VI) and Th(IV) from Sand with Neat                            Supercritical CO.sub.2 Containing TTA, TBP and TBP + TTA                      at 60° C. and 150 Atmospheres                                                  Ligand       Percent Extraction (%)                                   Ligand    Amount (μmole)                                                                            U        Th                                          ______________________________________                                        TTA       0              72 ± 4                                                                              74 ± 5                                   TBP       80             15 ± 4                                                                              10 ± 3                                   TBP + TTA 40 + 40        94 ± 5                                                                              93 ± 5                                   ______________________________________                                         *Each sand sample was about 200 mg and contained about 10 μg of Th.   

EXAMPLE XX

Uranyl ions also have been extracted from natural aqueous samples usingthe mixed ligand approach. The aqueous samples were mine waterscollected from the Northwest region. The uranium concentrations in twomine waters tested were 9.6 μg/mL and 18 μg/mL. The mine waters wereextracted with a 1:1 molar mixture of TTA+TBP in neat CO₂ at 60° C. and150 atm for a static time of 10 minutes followed, by 20 minutes ofdynamic extraction. Under these conditions, the percent extraction ofuranium from these samples were 81±4% and 78±5%, respectively, fortriplicate runs (Table XII).

The contaminated mine waters also were added to a top-soil samplecollected from northern Idaho. The contaminated soil samples were driedat room temperature prior to conducting the SFE experiments. The resultsof the extraction of uranium from the contaminated soil samples with a1:1 mixture of TTA+TBP or HFA+TBP in supercritical CO₂ at 60° C. and 150atm also are given in Table XII. The percent extraction of uranium withHFA+TBP for both soil samples A and B is about 90%, whereas TTA+TBPshows lower percent extractions (77-82%) of uranium under the sameconditions.

                  TABLE XII                                                       ______________________________________                                        Extraction of Uranyl Ions form Mine Waters                                    and From Contaminated Soils                                                                                     Percent                                                                       Extraction                                  Sample    U Concentration                                                                            Ligands    (%)                                         ______________________________________                                        Mine Water A                                                                             9.6 μg/mL                                                                              TTA + TBP  81 ± 4                                                          TTA + TOPO 97 ± 3                                   Mine Water B                                                                            18.0 μg/mL                                                                              TTA + TBP  78 ± 5                                   Soil A     6.3 μg/100 mg                                                                          HFA + TBP  91 ± 4                                                          TTA + TBP  82 ± 5                                                          HFA + TOPO 94 ± 5                                   Soil B    15.4 μg/100 mg                                                                          HFA + TBP  89 ± 5                                                          TTA + TBP  77 ± 4                                                          HFA + TOPO 98 ± 3                                   ______________________________________                                         *For mine water extractions: 4 ml sample, 200 mg each of TTA and TBP; For     the soil extractions: 100 mg sample, 200 μmole each of TTA and TBP or      of HFA and TBP. The extraction conditions were about 10 minutes of static     extraction, followed by 20 minutes of dynamic extraction                 

The examples provided above demonstrate that uranyl and Th(IV) ions,whether dissolved in an aqueous solvent or adsorbed on solid materialssuch as a cellulose support or sand, can be efficiently extracted usingsupercritical CO₂ containing a halogenated β-diketone, preferably afluorinated β-diketone. A binary mixture consisting oftri-n-octylphosphine oxide (TOPO) and a fluorinated β-diketone isslightly more effective than a binary mixture of TBP and a fluorinatedβ-diketone for the extraction of uranyl and Th(IV) ions from the minewaters and contaminated soil samples as shown in Table XII. Thestructure of TOPO is given below, wherein R₆ -R₈ are n-octyl groups,although it will be understood by those skilled in the art that R₆ -R₈also may be selected from the group consisting of lower alkyl groups.##STR21##

Uranium and thorium are usually extracted from these solids using strongacids for dissolution followed by various separation techniques. Toenhance the dissolution of actinides from solids with complex matrices,fluorinated acids may be used in supercritical CO₂. The matrixinterferences may be minimized using selective chelating agents. Thenovel SFE technique, which uses fluorinated β-diketones as extractants,and the synergistic effects obtained with trialkyl phosphates, offersnumerous applications for the separation of metal and metalloid ionsfrom solid and liquid materials.

EXAMPLE XXI

This example describes the extraction of lanthanides [such as La(III),Eu(III) and Lu(III)] from a solid matrix using supercritical carbondioxide and a binary mixture of TBP and a fluorinated β-diketone. Thefluorinated β-diketones were obtained from Aldrich Chemical Company.Solutions of La³⁺, Eu³⁺ and Lu³⁺ were prepared from the nitrate salts,which were obtained from Aldrich. Solid samples were prepared by spiking10 μg each of a mixture of the ions on prewashed filter paper or sand.The spiked samples were allowed to air dry at room temperature. Theweight of the dry sand samples was about 300 mg. All the experimentswere conducted using the apparatus of FIG. 1 and as described in example1.

Table XIII shows the results of extracting La³⁺, Eu³⁺ and Lu³⁺ from sandusing neat supercritical carbon dioxide containing TTA, TBP and mixedTTA/TBP at 60° C. and 150 atm. Synergistic extraction of the lanthanidesis again observed in this system when TTA is mixed with TBP. Theextraction efficiencies of the La³⁺, Eu³⁺ and Lu³⁺ in sand are 91%, 92%and 95%, respectively, when the carbon dioxide contained an equal molarmixture of TBP and TTA. The percent extraction represents the amount ofa lanthanide removed from the solid matrix. The percent recoveryrepresents the amount of the lanthanide trapped in the chloroformsolution.

                  TABLE XIII                                                      ______________________________________                                        Percent Extraction and Recovery of LA.sup.3+, Eu.sup.3+, and Lu.sup.3+        from Sand* with Neat CO.sub.2 Containing TTA, TBP, and                        Mixed TTA + TBP at 60° C. and 150 Atm                                  amt       percent extraction (%)                                                                        percent recovery (%)                                ligand                                                                              (μmol)                                                                             La      Eu    Lu    La    Eu    Lu                              ______________________________________                                        TTA   80      40 ± 3                                                                             51 ± 3                                                                           65 ± 4                                                                           29 ± 3                                                                           40 ± 3                                                                           60 ±                                                                       4                               TEP   80       4 ± 2                                                                              3 ± 1                                                                            5 ± 2                                                                            2 ± 1                                                                            2 ± 1                                                                            3 ±                                                                       1                               TTA + 40 + 40 91 ± 3                                                                             92 ± 4                                                                           95 ± 4                                                                           91 ± 3                                                                           89 ± 4                                                                           91 ±                         TBP                                           4                               ______________________________________                                         Each sand sample (300 mg by weight) contains 10 μg each of La.sup.3+,      Eu.sup.3+, and Lu.sup.3+. 10 min static extraction followed by 20 min of      dynamic extraction.                                                      

EXAMPLE XXII

This example describes the extraction of lanthanides [such as La(III),Eu(III) and Lu(III)] from an aqueous matrix using supercritical carbondioxide and a binary mixture of TBP and a fluorinated β-diketone. Thefluorinated β-diketones were obtained from Aldrich Chemical Company.Solutions of La³⁺, Eu³⁺ and Lu³⁺ were prepared from the nitrate salts,which were obtained from Aldrich. For the extraction of lanthanides fromwater, 4 ml of the spiked water sample was placed in a liquid extractionvessel. The pH of the solutions was controlled by an acetate buffer. Thewater sample typically contained about 2.5 μg/ml each of La³⁺, Eu³⁺ andLu³⁺. About 50 mg of a ligand, such as TTA, was loaded in the ligandcylinder placed upstream from the liquid extraction vessel. The sampleswere extracted dynamically at 60° C. and 150 atm. for a period of about20 minutes. When the extraction was complete the sample was removed andanalyzed by NAA. A standard solution containing 2.5 μg/ml each of thelanthanide ions was irradiated and counted with the sample underidentical conditions.

The results of these extractions are provided below in Table XIV.Without the ligand, the lanthanide ions in the aqueous phase cannot beextracted (les than about 2%) by supercritical carbon dioxide even with5% methanol present in the fluid phase. With TTA present in the fluidphase, the extraction efficiencies of the La³⁺, Eu³⁺ and Lu³⁺ are 30%,38% and 51% percent respectively. Using methanol as a modifying solventincreased the extraction efficiencies to 70%, 78% and 81%.

The synergistic extraction of the lanthanides from the aqueous solutionby a mixture of TTA and TBP in supercritical carbon dioxide also isillustrated in Table XIV. TBP alone in supercritical carbon dioxideshows low extraction efficiencies for the lanthanides (less than about12%). When a mixture of TBP and TTA is used the extraction efficienciesare increased to 75%, 86% and 89% for La³⁺, Eu³⁺ and Lu³⁺, respectively.

                  TABLE XIV                                                       ______________________________________                                        Percent Extraction of La.sup.3+, Eu.sup.3+, and .sup.3+  from                 Aqueous Solution with Supercritical CO.sub.2 Containing                       TTA at 150 Atm and 60° C.                                                             percent extraction (%)                                         ligand   fluid phase La.sup.3+ Eu.sup.3+                                                                           Lu.sup.3+                                ______________________________________                                        none     CO.sub.2    <2        <2    <2                                       none     CO.sub.2 + 5% MeOH                                                                        <2        <2    <2                                       TTA      CO.sub.2    30 ± 3 38 ± 4                                                                           51 ± 4                                TTA      CO.sub.2 + 5% MeOH                                                                        70 ± 3 78 ± 4                                                                           81 ± 3                                TBP      CO.sub.2     7 ± 2 11 ± 3                                                                           12 ± 3                                TBP + TTA                                                                              CO.sub.2    75 ± 3 86 ± 4                                                                           89 ± 3                                ______________________________________                                         Each water sample (4 mL) contains 2.5 μg/mL of La.sup.3+, Eu.sup.3+,       and Lu.sup.3+  each at pH 4.0 controlled by an acetate buffer.           

G. CO₂ SFP of Metals and Metalloids from Acidic Solutions

One specific embodiment of the present invention is the removal ofradioactive ions from solid and liquid materials. For instance, thedescribed SFE process can be used to extract actinides in acid solutionssuch as those produced by the PUREX process (Plutonium Uranium Recoveryby Extraction). In the PUREX process, nuclear fuel material is firstdissolved in hot nitric acid followed by extraction of the dissolveduranian and plutonium with an organic solvent containing 20-30% of TBPin kerosene or in n-dodecane. Supercritical fluid extraction of metalsand metalloids in acid solutions using carbon dioxide also has provenuseful.

EXAMPLE XXIII

This example concerns the extraction of U(VI), Th(IV) and Nd(III) from 6molar nitric acid (HNO₃) . The procedure used for the extraction wassubstantially as described above in example XIV. The ligands used forthis example included TBP and TTA, as well as TOPO and TTA. For the TBPexperiments, the ligand (5ml TBP) was placed in a stainless steel vesselwith supercritical carbon dioxide bubbled from the bottom of the vessel.In this arrangement, the fluid phase is saturated with TBP. For theextractions with a binary mixture of TBP and TTA, supercritical carbondioxide was first saturated with TBP using the method described above.The fluid phase then passed through a second ligand cell containing 100mg of TTA. After this, the supercritical fluid, saturated with both TBPand TTA, was led into a liquid extraction cell from the bottom. Theextraction conditions were 15 minutes static extraction followed by 15minutes of dynamic extraction at 60° C. and 150 atm. The supercriticalconditions were 60° C. and 150 atmospheres. The results of theseextractions are presented below in Table XV, which shows that the mixedligand composition provides a novel and efficient means for removingradioactive ions from acidic solutions, without using toxic or flammableorganic chemicals for the extraction process. Thus, the presentinvention provides an attractive alternative for the PUREX process.

                  TABLE XV                                                        ______________________________________                                        Extraction of U(VI), Th(IV), and Nd(III) from 6 M Nitric                      Acid with Supercritical CO.sub.2 and Mixed Ligands                                       % Extraction                                                       Ligands      U            Th    Nd                                            ______________________________________                                        TBP          91           89    67                                            TBP + TTA    95           82    75                                            TBP + TTA    97           91    77                                            TOPO + TTA   99           99    73                                            ______________________________________                                         a. Sample composition: 50 μg/mL each of U, Th, and Nd in 6M HNO.sub.3      +3M LiNO.sub.3                                                                b. Extraction conditions: 15 min static plus 15 min dynamic extraction at     60° C. and 150 atm.                                               

This example and the results provided in Table XV show that TBP alonecan extract uranyl, Th(IV) and Nd(III) in 6 m HNO₃ with a reasonablyhigh efficiency at 600° C. and 150 atm. Without limiting the extractionof metal and/or metalloid ions from acidic solutions to one theory ofoperation, it appears that these ions are extracted as the neutralnitrates UO₂ (NO₃)₂, Th(NO₃)₄ and Nd(NO₃)₃ in supercritical carbondioxide because of the high nitrate concentration in 6 M HNO₃. Using abinary mixture of TBP and the fluorinated β-diketone TTA, the extractionefficiencies of uranyl, Th(IV) and Nd(III) in the acid solution bysupercritical carbon dioxide are enhanced. These results show that theactinides and lanthanides (Nd(III) is a typical lanthanide) can beextracted from acid solutions, particularly nitric acid solutions, on anindustrial scale using supercritical carbon dioxide as a solvent.

In summary, example XXII shows that lanthanides and actinides can beextracted from acidic solutions, such as 6 M HNO₃, using TBP insupercritical carbon dioxide. A binary mixture of TBP and a fluorinatedβ-diketone, or TOPO and a fluorinated β-diketone, in supercriticalcarbon dioxide can enhance the extraction efficiencies of thelanthanides and the actinides from acidic solutions. As a result, carbondioxide can be used, either in a supercritical state or as a subcriticalliquid under pressure, to replace the use of organic solvents, such askerosene, for the extraction of lanthanide and actinides from acidicsolutions. The triple point of carbon dioxide is 5.1 atm and -56.3° C.Therefore, at room temperature carbon dioxide becomes a liquid above 5.1atm. Depending on the pressure, liquid carbon dioxide has a densitycomparable or slightly greater than supercritical carbon dioxide, thusthe solvation power of liquid carbon dioxide is comparable to that ofsupercritical carbon dioxide. This means liquid carbon dioxide shouldalso be able to dissolve the metal complexes described above. However,liquid carbon dioxide does not have the "gas-like" properties of thesupercritical carbon dioxide. This means liquid carbon dioxide has largea viscosity, small diffusivity, and consequently poor penetration powercompared with the supercritical carbon dioxide. Thus, it is expectedthat liquid carbon dioxide should also be able to extract lanthanidesand actinides from acid solutions with TBP or a mixture of TBP and afluorinated β-diketone as the extractant, but with lower efficiencies.The extraction efficiency of liquid carbon dioxide is expected to dependon the applied pressure. It is also expected that the extractionefficiency of liquid carbon dioxide can be improved with mechanicalstirring and agitation.

Having illustrated and described the principles of the invention inseveral preferred embodiments, it should be apparent to those skilled inthe art that the invention can be modified in arrangement and detailwithout departing from such principles. We claim all modificationscoming within the spirit and scope of the following claims.

I claim:
 1. A method for extracting a metalloid or metal species from asolid or liquid, comprising exposing the solid or liquid to asupercritical fluid solvent containing a β-diketone chelating agent fora sufficient period of time to form chelates between the agent andspecies that are solubilized in the supercritical fluid solvent.